Purification of petroleum coke

ABSTRACT

PETROLEUM COKE IS PURIFIED OF SULFUR AND METALLIC IMPURITIES BY HYDRODESULFURIZATION WITH SYNTHESIS GAS FOLLOWED BY PASSING THE RESULTING SYNTHESIS GAS, WHICH IS RELATIVELY RICH IN HYDROGEN SULFIDE, IN CONTACT WITH THE COKE, AT A TEMPERATURE AND PRESSURE AT WHICH THE HYDROGEN SULFIDE CATALYZES THE REACTION OF THE METALLIC IMPURITES WITH CARBON MONOXIDE TO FORM GASIFORM METALLIC CARBONYLS WHICH CAN BE PHYSICALLY SEPARATED FROM THE COKE.

Aug. 10, 1971 w. F. FRANZ ETAL 3,598,528

PURIFICATION OF PETROLEUM COKE Filed June 27, 1969 United States Patent Oiiice 3,598,528 PURIFICATION OF PETROLEUM COKE William F. Franz, Gardiner, and Howard V. Hess, Glenham, N.Y., assignors to Texaco Inc., New York, N.Y. Filed June 27, 1969, Ser. No. 837,254 Int. Cl. Clllb 3]/02 U.S. Cl. 23-209.9 6 Claims ABSTRACT OF THE DISCLOSURE Petroleum coke is purified of sulfur and metallic impurities by hydrodesulfurization with synthesis gas followed by passing the resulting synthesis gas, which is relatively rich in hydrogen sulfide, in contact with the coke at a temperature and pressure at which the hydrogen sulfide catalyzes the reaction of the metallic impurities with `carbon monoxide to form gasiform metallic carbonyls which can be physically separated from the coke.

The present invention relates to the up-grading and purification of petroleum coke by the removal of typical impurities and contaminants which limit the use f this product in desirable specialty applications.

BACKGROUND OF INVENTION The requirement for calcined coke for metallurgical uses; as for example, in the manufacture of electrodes for use in the aluminum industry, imposes requirements not ordinarily met by untreated petroleum coke. These latter cokes, whether made by fluid or by the delayed coking process, or other conventional methods, tend to concentrate the undesirable constituents of the petroleum, such as residual sulfur and metals such as iron, nickel or vanadium.

This follows from the fact that coking itself offers a desirable means for obtaining valuable fractions from rather low grade residua. Moreover the process of coking itself, inherently tends to concentrate the undesirable constituents of the residual oil in the coke.

It has been proposed to remove sulfur from petroleum coke by high temperature hydrogenation. For example, in Industrial Engineering Chemistry, September 1959, volume 51, No. 9, page 1027 hydrodesulfurization at elevated temperatures in the vicinity of 1300 F. and at high hydrogen rates is preceded by a restricted, low temperature -preoxidation treatment, which is said to open up the structure of the coke and to increase the surface area thereof in such a way as to promote and facilitate the hydrodesulfurization reaction. This treatment, however, fails to materially reduce the level of metal contaminants which are also seriously objectionable.

PRESENT INVENTION The present invention proposes to effect the purification, and the substantial reduction in the level of both the aforementioned sulfur and metallic impurities in a high sulfur-containing coke by successive treatments of the coke with synthesis gas, first to effect hydrodesulfurization and thereafter to remove said metals as valuable carbonyls.

More specifically, the present invention proposes to preheat and preoxidize the petroleum coke followed by hydrodesulfurization of the preoxidized coke through intimate contact with a stream of synthesis gas rich in hydrogen at a temperature in the range of about 1300-1800 F., preferably at the lower end of this range in the region of about 1300 F. Under these conditions the molecular hydrogen of the synthesis gas reacts with the contaminant sulfur of the coke to form hydrogen sulfide which passes olf with the effluent stream of synthesis gas,

Thereafter the coke is demetallized, that is to say, the metallic contaminants are separated, by intimate contact with the aforesaid efiluent stream of synthesis gas at a substantially lower temperature in the range of, about 275-430 F., at which carbon monoxide of the synthesis gas selectively reacts with the metals to form the respective carbonyls.

An important feature of the present invention involves conducting the carbonylation reaction at improved rates of reaction in the presence of the synthesis gas containing the sulfur derived from the previous step, of hydrodesulfurization as hydrogen sulfide, which has a catalytic effect upon the carbonylation reaction. Actually it has been found, for example, that the formation of iron pentacarbonyl becomes quite rapid in the presence of only a little hydrogen sulfide.

The advantages contemplated by the present invention therefore involve the general facilitation and acceleration of the several reactions to 4enable coke purification at commercially feasible rates.

The present invention requires for the treatment of the coke, no treatment agent other than a stream of synthesis gas, which is increasingly becoming a more important and necessary requirement for petroleum refineries under sharply demanding hydrogen requirements. Therefore the incoming stream of synthesis gas makes available a large excess of treating material which is favorable to a thoroughgoing desulfurization and demetalization of the coke. Furthermore this can be employed without any appreciable consumption of the synthesis gas, that is to say, without materially penalizing the final yield of synthesis gas which goes on to the refinery.

The invention also provides, as a byproduct, a convenient source of iron, nickel and vanadium in a highly specialized form, desirable for parti-cular use, namely as a high grade Raney metal. Finally the incidental calcination of the coke becomes an important feature of the product.

DEFINITIONS The term synthesis gas as used herein means gaseous mixtures consisting essentially of molecular hydrogen and carbon monoxide, such as are produced either by partial oxidation or steam reforming of hydrocarbons including coke gasification.

The term high-sulfur coke means a petroleum coke having a sulfur content of at least 4% and typically in the range of about 5-8% or higher.

The terms desulfurization and hydrodesulfurization, as employed herein, are intended to include partial reduction of the sulfur content as well as substantially complete removal of sulfur from the coke. Likewise the Words demetalization or carbonylation as used herein are concerned with partial as well as entire removal of the metal, since it is understood by those skilled in the art that the foregoing processes all desirably accomplish a reduction in impurity content of the material undergoing treatment but seldom, if ever, accomplish complete removal of the material.

DETAILS` OF PRESENT INVENTION As previously indicated the hydrodesulfurization step is conducted at elevated temperatures of about 1300 F. and above, preferably with a large excess of synthesis gas over and above the amount stoichiometrically required for reaction with all of the sulfur. While it is not anticipated that pressure will materially facilitate the reaction, pressures up to 50()` p.s.i.g. may be employed.

The metal carbonylation step is conducted at a temperature in the range of about 275-430 F., preferably also with the treating gas in substantial excess. Inasmuch as this reaction is favored by pressure, it is preferred to conduct this step at pressures above atmospheric ibut in the range at which the product carbonyls remain in the gasiform or volatile phase and thus are free to pass of and separate from the coke. In specific terms therefore the pressure range is preferably substantially above atmospheric and advantageously in the range of 450-750 p.s.i.g'. although, depending on the content and disposition of the metallic impurities, the pressure may go as high as 1000 p.s.i.g.

It is important to note that hydrodesulfurization of the coke can be conducted simultaneously with coke gasification at temperatures, for example, in the range of 1300- 1800 F. or higher. yBy gasification is meant the reaction of a portion of the coke with molecular oxygen and steam to produce synthesis gas. Accordingly, therefore gasificationv of the coke in this manner provides a stream of synthesis gas at the proper temperature for conducting desulfurization.

The accompanying drawing is a diagrammatic representation of a series of successive process steps illustrating a preferred embodiment of the invention.

In referring to this drawing it is to be specifically understood that it is not intended to represent a continuous process but rather a batch operation in which each of the indicated steps stands for a separate and successive treatment of the coke progressing serially from left to right as in the figure. Accordingly, the invention contemplates the successive switching of dow (not shown but understood by those skilled in the art) so that each batch of coke is successively subjected to each successive process step as the preceding process step is completed.

In preheating step 10 the loose mass of particulate coke is continuously contacted by a hot upowing stream of synthesis gas from the hydrodesulfurization step to be hereinafter described more in detail. Hot synthesis gas comes into the coke preheater 10 via line r12 at a ternperature, for example, in the neighborhood of 1300 F. or above, gradually bringing the coke temperature up to the level of 60G-800 F., preferably 650`700 F.

To facilitate the preheating as well as the succeeding steps to be hereinafter described, the coke mass preferably takes the form of a lfiuidized bed of fine granules, for example, 35-60 mesh or finer and with the synthesis gas input flowing upwardly at a rate sufficient to maintain particles in the dense fluid phase.

With the coke preheated to about 700 F. the initial batch is now switched to the second step indicated by reference numeral 14 wherein controlled oxidation is effected by an upow of molecular oxygen or air introduced via line 16. Preoxidation, as previously intimated, calls for carefully limited low temperature oxidation below 1100 F. and preferably in the neighborhood of about 650 F., such that only about 649% of the coke is consumed. lOne observable result is a sharp increase in the effective surface area of the coke which rises typically from about up to above 200 sq. meters per gram and normally in the region of 400450 sq. meters per gram. While this does not appear to be the sole or exclusive source of the improved subsequent reaction rates which have been observed, it seems to be one factor which contributes to effecting subsequent chemical reactions at such improved rates.

The eliiuent gases from the preoxidation step withdrawn through pipe 18, consist essentially of carbon monoxide (and unaltered nitrogen when air is used to conduct the operation). This stream can be discarded or can be mixed with the effluent gas from the coke preheater in pipe 30 which, as previously indicated, is essentially synthesis gas containing hydrogen sulfide, at a temperature in the neighborhood of 700 F.

Actually the addition of the efiiuent gas from pipe 18 to the synthesis gas in pipe 30 is to be preferred when essentially pure oxygen is introduced in pipe 16 in controlled amounts, such that pipe 18 delivers essentially pure CO.

The following step of hydrodesulfurization, 20, vis conducted with the fresh synthesis gas from coke gasifier 22 maintained at a suitable gasification temperature as, for example, 1600-2400 F., and supplied with a mixture of oxygen and steam via line 24. Accordingly, the effluent gases from pipe 26, consist essentially of hydrogen and carbon monoxide, at the temperature of the coke gasification step. These therefore maintain the hydrodesulfurization unit 20 at a temperature level of 1300-1600o F., preferably at about the former temperature, such that the sulfur content of the coke is converted into hydrogen sulfide which passes off lwith the efliuent gas stream 12.

It is therefore this synthesis gas stream relatively rich in hydrogen sulfide at a temperature of about 1300 F., which transfers its sensible heat to the fresh coke in the preheating step 10.

It is likewise this same gas at a reduced temperature, as for example 60G-700 F., which passes from the preheater 10 through pipe 30 to effect carbonylation at 32. As previously indicated, the carbonylation takes place at a temperature of 275-430 F. and a pressure preferably about 450-750 p.s.i.g. developed by pump 34. Accordingly the effluent stream of gases Withdrawn through pipe 36 comprises hydrogen, carbon monoxide, the metal carbonyls and hydrogen sulfide catalyst, which pass on to a suitable unit 38 for recovering the metal content of the gaseous stream.

These may be recovered as individual carbonyls or, as in the present embodiment, by fractional separation o f the various metal carbonyls and decomposition at elevated temperatures to form relatively pure metal products, namely nickel, iron and vanadium, delivered respectively at 40, 42 and 44.

The resulting stream of synthesis gas 46 passes into shift converter where it is reacted with steam introduced as at 48 to convert contained carbon monoxide into carbon dioxide, which is subsequently scrubbed out ofthe gas stream together with the hydrogen sulfide gas. Thus a pure stream of hydrogen is available at at 52, whereas by-product hydrogen sulfide and carbon dioxide are delivered respectively via lines S4 and 56.

The details of metal recovery, shift conversion and the scrubbing operation are not given herein because they are well known processes which are operated in accordance with established and accepted principles.

Furthermore, reference is made to our copending appllcation Ser. No. 837,255 filed on June 27, 1969 for a more detailed representation of the steps.

Instead of coke gasification, the original synthesis gas may be derived from any other suitable source, such as for example partial oxidation of a petroleum fraction the products of which enter via the dotted line conduit 60, are preferably heat exchanged as at 62 with the hot effluent gases in line 12 and thereafter flow through line 64 and heater 66 to the hydrodesulfurization reactor line 68 at a temperature which maintains the aforementioned operating conditions therein.

The present invention therefore proposes preoxidation of a high sulfur coke containing typical excessive amounts of metallic impurities, first to open up its structure and facilitate subsequent reaction, followed by hydrodesulfurization in a fresh stream of synthesis gas containing, for example, equal parts of hydrogen and carbon monoxide. The resulting synthesis gas, now containing the original sulfur in rgaseous form, after transferring its sensible heat to the incoming batch of coke, catalytically carbonylates the desulfurized coke with hydrogen sulfide as the catalyst. Good rates of reaction are anticipated as a result of the successive prior treatments, plus the catalytic effect of hydrogen sulfide upon the carbonylation reaction.

We claim:

1. In the purification of a high sulfur containing coke which also contains objectionable proportions of metals, such as nickel, iron and vanadium to effect desulfuriza tion, demetalization and calcination thereof to form metallurgical grade coke, the steps which comprise preoxidizing said coke in the region of G50-850 F. to effect a substantial increase inthe surface area thereof,

subjecting the resulting coke to intimate Contact with synthesis gas at a temperature of about 1300" F. or above to effect hydrodesulfurization of said coke with the production of an effluent gas containing hydrogen sulfide, effecting catalytic demetalization of the coke by intimate contact between said efuent synthesis gas and said desulfurized coke at a temperature in the range of about 275-430 F. and under an elevated pressure at which said metals catalytically react with carbon monoxide to form, with said hydrogen su1 de as catalyst, gasiform carbonyls of said metals,

and separating said gasiform phase containing said metal carbonyls, carbon monoxide, hydrogen and hydrogen sulfide from said coke.

2. The purication of a high sulfur petroleum coke as called for in claim 1 wherein at least a portion of the sensible heat of the synthesis gas effluent from the hydrodesulfurization is employed to preheat the coke prior to preoxidation.

3. The purification of a high sulfur petroleum coke as called for in claim 1 wherein said carbonylation step is conducted at a pressure substantially above atmospheric in the range at which said carbonyls of said metals remain in gasiform phase.

4. The purification of high sulfur petroleum coke as called for in claim 1 wherein said carbonylation step is conducted at a pressure in the range of about 450-750 p.s.1.g.

5. The purification of high sulfur petroleum coke as called for in claim 1 wherein said hydrodesulfurization step is conducted in the presence of hydrogen formed by simultaneously gasifying said coke at a temperature of about 1300-1800 F. by contact with a mixture of molecular oxygen and steam.

6. The purication of high sulfur petroleum coke as called for in claim 1 wherein said hydrodesulfurization is conducted in the presence of a stream of synthesis gas at a temperature about 1300 F.

References Cited UNITED STATES PATENTS 2,721,169 10/1955 Mason et al. 201-17 3,226,316 12/1965 Metrailer et al. 23-209,9X

OTHER REFERENCES Mason: Ind. & Eng. Chem., vol. 5l, September 1959, pages 1027-1030.

EDWARD I. MEROS, Primary Examiner U.S. Cl. X.R. 

